Polypropylene is superior in properties such as transparency, heat resistance and food hygienic property, and therefore extensively used in a field such as a film including a sheet. In recent years, high-speed packaging has increasingly advanced in a field of food packaging, and therefore a film having a good low temperature heat seal property has been desired.
As a film having a good low temperature heat seal property, for example, Japanese Patent No. 2,882,237 discloses a film comprising a polypropylene random copolymer, which is obtained by copolymerizing propylene and an α-olefin or copolymerizing propylene, ethylene and an α-olefin substantially in the absence of a solvent using a Zieglar Natta catalyst, wherein respective contents of a propylene unit, an ethylene unit, an α-olefin unit and a 20° C. xylene soluble portion are specified, and a unit of a comonomer such as the α-olefin and ethylene is increased in its content. However, the film has a problem that stiffness property and food hygienic property decrease when the comonomer unit content exceeds a certain degree. In the present invention, the term such as “propylene unit” means a polymerization unit of a monomer such as a polymerization unit of propylene.
Further, Japanese Patent Publication No. 2-57770 and Japanese Patent No. 3,070,419 disclose a polypropylene laminated film, which contains an outer layer comprising a polypropylene copolymer having an extremely high content of an α-olefin unit. However, the laminated film has a problem that its heat seal property deteriorates owing to corona treatment, and another problem that heat seal temperature cannot be sufficiently lowered due to occurrence of stickiness in a film formation step.